Docs: Build crash due to Google Analytics configuration

[BlueRexPY]

Build crash due to Google Analytics configuration:

[ERROR] Error: The "gtag" field in themeConfig should now be specified as option for plugin-google-gtag. For preset-classic, simply move themeConfig.gtag to preset options. More information at https://github.com/facebook/docusaurus/pull/5832.
    at preset (/home/admin/Projects/JS/boilerplates/superplate/documentation/node_modules/@docusaurus/preset-classic/lib/index.js:27:15)
    at /home/admin/Projects/JS/boilerplates/superplate/documentation/node_modules/@docusaurus/core/lib/server/plugins/presets.js:39:62
    at Array.forEach (<anonymous>)
    at loadPresets (/home/admin/Projects/JS/boilerplates/superplate/documentation/node_modules/@docusaurus/core/lib/server/plugins/presets.js:25:13)
    at loadPluginConfigs (/home/admin/Projects/JS/boilerplates/superplate/documentation/node_modules/@docusaurus/core/lib/server/plugins/configs.js:72:52)
    at initPlugins (/home/admin/Projects/JS/boilerplates/superplate/documentation/node_modules/@docusaurus/core/lib/server/plugins/init.js:42:65)
    at loadPlugins (/home/admin/Projects/JS/boilerplates/superplate/documentation/node_modules/@docusaurus/core/lib/server/plugins/index.js:26:50)
    at load (/home/admin/Projects/JS/boilerplates/superplate/documentation/node_modules/@docusaurus/core/lib/server/index.js:76:90)
    at async Command.start (/home/admin/Projects/JS/boilerplates/superplate/documentation/node_modules/@docusaurus/core/lib/commands/start.js:44:19)

[BlueRexPY]

Fix - #385